Mastering Lithium Niobate Modulators: Essential Design & Theory

Hi everybody, today I would like to discuss the simulation of a lithium niobate modulator. My name is Majid Ebna Ali Hidari, and I am the Technical Manager of Photonics at Ozen Engineering, Inc., an ANSYS early channel partner.

Outline of the Talk

1. Structure of the Modulator
2. Workflow of the Simulation
3. Calculating the Perturb Index on Pockel's Effect
4. Main Parameters and Modulation Merits
5. Live Demo of Electrical and Optical Simulation

Structure of the Modulator

The modulator consists of the following components:

• Quartz
• Silica
• Lithium Niobate
• Two ground electrodes and one signal electrode

In the software, I will demonstrate how to design these parameters using numerical tools.

Workflow of a Lithium Niobate Electro-Optic Modulator

First, you need to work with ANSYS Charge, which is part of ANSYS Multi-Physics. In this process, you will:

• Sweep the voltage and calculate the spatial electric fields (E-fields).
• Calculate the Pockel's effect to determine the perturbed index.
• Extract values such as n versus voltage in the x and y directions.

These values are then used in ANSYS Theme to solve the TE₀ mode for all voltages and evaluate modulator performance merits like loss and modulator length.

Calculating the Perturbed Index

To calculate the perturbed index using the Pockel's effect, we need the full tensor for lithium niobate. The formula requires parameters such as E_x, E_y, E_z, and constants like r₃₃, r₄₂, r₁₃, and r₂₂. In this case, E_y is zero, and E_x is much larger than E_z, allowing us to simplify the formula.

To calculate the perturbed index, we need:

• Extraordinary and ordinary values of the effective refractive index (n_e and n_o)
• Constants r₃₃ and r₁₃

With these values, the software calculates E_x, allowing us to determine how much the effective index changes with voltage.

Modulation Merits

In the numerical field, we define modulation merits such as:

• L_π: Modulation length required for a phase shift of π.
• Loss parameter, which depends on the imaginary part of the effective refractive index (κ).

By changing the voltage, the modulation length decreases, while the loss value remains constant. This is a simpler structure for the lithium niobate modulator, but more complex structures can be designed, such as photonic crystal structures, Bragg grating structures, and slot waveguides. Lithium niobate can also be integrated with silicon.

I have published an article on this topic on the Ozen website, where I discuss these details and present two case studies for the lithium niobate modulator. For more information, please visit our website and knowledge base article.

Next Steps

In the next step, I will demonstrate how to use Numerical Charge for electrical simulation and Numerical Theme for optical simulation. Thank you for your attention, and I look forward to seeing you there.